1. Field of the Invention
This invention relates generally to apparatus for reproducing signals recorded on a disc, and more particularly is directed to an improved digital audio disc player or reproducing apparatus.
2. Description of the Prior Art
Apparatus exists to reproduce or play back a digital audio disc, sometimes referred to as a compact disc, having digitized signals representing audio information and also disc position information recorded in successive, substantially circular tracks which may be successive turns of a continuous spiral path on the record disc or concentric circular tracks having respective different diameters. In one existing type of reproducing or playback apparatus for a digital audio disc, the latter is rotated by a spindle motor while a pickup device scans the disc and is moved radially, usually from the inner periphery toward the outer periphery of the disc surface, by a transporting mechanism which operates in synchronism with the rotation of the disc by the spindle motor. Further, a signal indicating the radial position of the pickup device is used to control the rotational speed of the spindle motor so that the rotational speed of the digital audio disc is varied to provide a constant linear speed at the position on the disc being scanned by the pickup device at all times.
In the case of a digital audio disc having a number of musical selections recorded in various respective groups of tracks on the disc, it is the usual practice to provide unrecorded areas, or at least areas that are not recorded with audio information, between the successive recorded musical selections. In a digital audio disc player of the prior art, when a desired musical selection is to be reproduced from a desired position at the beginning or within such music selection, a playback standby state is established in which the pickup device repeatedly scans or traces a single turn of the spiral path defining the successive substantially circular tracks and which terminates shortly before the desired position at which reproducing of the recorded digitized signals is to commence. If the digital audio disc player is released from such playback standby state in response to a play command signal, an indeterminant or variable delay occurs between such play command signal and the actual commencement of reproduced music or audio information. For example, in the case of a compact digital audio disc, the time required for the pickup device to trace or scan one track varies from 0.126 second, in the case of a track at the inner periphery of the recorded area of the disc, to 0.292 second, in the case of a track at the outer periphery of the recorded disc area. Accordingly, there will be a variable delay ranging up to a maximum of 0.292 second from the occurrence of the play command signal until the commencement of actual reproduction of audio information or music, with the extent of such delay depending upon the radial position on the disc then being scanned by the pickup device, and also on the rotational or angular distance between the point on the disc being scanned by the pickup device at the time when the play command signal occurs and the circumferential position along a track at which the desired music selection actually commences. Although such delay of indefinite length does not pose a serious problem in the general use of the digital audio disc, it may be quite disadvantageous when digital audio discs are employed at broadcasting stations, in which case so called "dead time" in the broadcast transmission is to be avoided. Further, as a practical matter, digital audio discs have digitized signals recorded thereon for representing the playing time of following recorded musical selections. Therefore, the point of "playing time zero" which precedes each group of digitized playing time signals is not coincident with the actual commencement of the recorded audio or music signals. Thus, even if a music selection is played back precisely from its point of "playing time zero", a soundless period of indefinite length will occur prior to the actual playback of music, and such soundless period again causes a problem in the use of digital audio discs by broadcasting stations.